If you're keeping it casual and low-key:

Deciding who gets invited to your wedding brunch depends largely on the venue and the timing. If you're thinking you'd like a low-key affair at your parents' home, it's probably best to limit the guest list to just immediate family and your wedding party, emphasizing that this is a small get together and that they are under no obligation to attend. In this case, it's okay for the invitations to be by word of mouth, text, or personal phone call. This could get a little awkward if news spreads around to uninvited guests, so you may want to stress to the invitees that the gathering will be a small group.

If brunch will be hosted at the hotel or a special venue:

If your post-wedding brunch will be hosted at a hotel where many of your wedding guests are staying for the weekend, or at a local restaurant during a destination-wedding weekend, it's best to go ahead and invite all of your wedding guests. As you'll need a head count for catering and you'll want to give guests ample notice of the post-wedding brunch so they can make their travel plans accordingly, you should include this event with your wedding invitation. This is typically added in the form of an insert or weekend agenda card. You may also add brunch attendance on the reply card. In this case, you'll also want to re-iterate the information about brunch on your wedding website and in your wedding welcome letter if you send guests staying in hotels a welcome bag.

Yes, you should probably attend.

A question commonly asked by the exhausted bride and groom: "Do we have to go to our post-wedding brunch?" Well, someone went to a great effort to put this event together and it's in your honor, so you'd be courteous to show up. That said, most guests will understand if you don't stay for long. Besides, you'll probably need some coffee and breakfast after all that late-night partying.